In the film , Buzz Lightyear is convinced he is not a toy but on a space ranger mission to seek out the evil Zurg..
So why does he freeze like all the other toys ...
What a question! I'll never watch Toy story without wondering now...
However, on IMDB it suggests..
''...One possible explanation is that it is an instinct for all toys, so Buzz does it involuntarily, much like falling asleep. A second possibility is that Buzz considers humans dangerous given their size, so he imitates toys because their behavior seems most appropriate...'
;-)